1. Technical Field
The present invention relates in general to an improved system and apparatus for receiving serially transmitted data, and in particular to a method and apparatus for determining byte synchronization between a received serial data stream and a data detector within a data processing system.
2. Description of the Related Art
When transmitting data serially, it is important that the sender and receiver of a serial data stream be in synchronization. When such synchronization between sending and receiving units is achieved, that condition may be referred to as obtaining byte synchronization, or "byte sync." Byte sync is required to correctly recognize and recover the transmitted data because recognizing the first bit of the first word of a customer data stream is necessary before the receiver is able to correctly interpret the data bits that follow, and correctly group those data bits into data words comprising the customer data.
Byte sync is achieved when the clock frequency and phase of the receiving unit has been aligned or synchronized with the data clock utilized to produce the data stream, and when the receiving unit has identified the first bit of the first word of the customer data portion of a serial data stream.
Since the reception of a customer data stream without being able to distinguish which data bits form a particular data word, or which word is the first word, is of no value, it is an important aspect of increasing transmission efficiency that a receiver's ability to obtain byte sync be at least as reliable as the transmission of the serial data stream containing customer data. While there are many known methods of improving transmission efficiency by detecting and correcting bit errors in the customer data portion of the serial data stream, such methods of improving efficiency are usually not available to improve the reliability of the portion of the serial data stream utilized to obtain byte sync. Methods available to detect and correct bit errors in the customer data portion typically include extra data, such as redundant parity check characters; but because the synchronization pattern at the beginning of a serial data stream typically comprises a relatively small number of bits, it may be impractical to add such extra data. Additionally, detecting and correcting errors takes time that may not be available while the receiver is attempting to locate the synchronization pattern utilized to obtain byte sync. Thus, it is important to reliably recognize the synchronization pattern to increase the overall efficiency of data transmission.
In a typical receiver, such a waveform serves as the input signal for an equalization circuit (i.e., an equalizer), and following the equalization circuit, a data detector circuit (i.e., a detector). The purpose of the equalizer is to reduce unwanted effects of overlapping signals which represent adjacent data bits, and to shape the waveform so that it conforms to a standard format for a particular type of detector. The effects of overlapping signals which represent adjacent data bits is referred to as intersymbol interference (ISI), and, in a partial response system, such ISI is utilized constructively by allowing ISI to occur in a controlled manner. The equalizer helps control ISI to maximize the probability that the detector will correctly detect data bits.
However, before some equalization circuits, such as adaptive equalizers, are able to control ISI, several equalizer coefficients utilized in the equalization process may need to be calculated. Typically, the calculation of equalizer coefficients occurs while receiving a known portion of a waveform, and many times, byte sync must occur before the equalizer can be trained by receiving a known waveform. If the waveform utilized to train the equalizer is known, including its placement in time, the calculation of equalizer co-efficients proceeds more accurately and rapidly than if the training waveform is not known. Thus, for best results, byte synchronization must occur before calculation of the equalizer co-efficients (i.e., training the equalizer).
The purpose of the detector is to produce a series of digital data bits by interpreting a periodic series of digitized samples representing the shape of the incoming waveform. By interpreting these digitized samples, the customer data may be correctly recovered from the incoming waveform. The accuracy of such data interpretation, however, is susceptible to the effects of misequalization, noise, distortion, DC offset, transients, electromagnetic interference, and the like. Recently, detectors have been designed that require an indication of byte sync before the detector is able to best interpret the digitized samples. Such detectors are called time-varying detectors.
Sources of noise include thermal or "Johnson" noise, and media noise (if the waveform is produced by reading magnetic or other media). Johnson noise is caused by the continuous movement of charged particles in the materials that make up electronic components. The source of media noise may be the granularity of the media surface, or variations in the surface of the magnetic media. Defects in the magnetic recording media may also cause variations in the amplitude of the input waveform. Additionally, a pin hole or dent in the magnetic media may cause the input waveform to disappear altogether.
Waveform distortion may result from misequalization, the addition of a DC offset, or transients. Misequalization may be caused by incorrectly setting equalizer coefficients, or, in the case of an adaptive equalizer, incorrectly training the equalizer circuit such that the equalizer coefficients are calculated incorrectly. A DC offset may be added to the waveform if the tolerance of circuit components drift and cause circuits to be out of adjustment. Transients may be caused by switching from a "writing" operation to a "reading" operation, or by switching from one read head to another read head in the disk drive unit.
Sources of electromagnetic interference include radiation from electric motors, electrostatic discharge, electric or magnetic fields, fluorescent lights, and the like. Two other sources of interference in a disk drive unit are previously recorded information which may not have been fully erased, and information recorded on adjacent tracks which is picked up by the read head because of inaccurate track following.
In the prior art, obtaining byte sync involves first receiving an analog waveform, which represents a serial data stream. An example of such a waveform is produced as a read head responds to flux changes on the surface of a magnetic storage media. Thereafter, such a waveform may be conditioned by an equalizer, and sampled by an analog-to-digital converter (ADC), to produce a data stream of data words having a standard format, such as, for example, the extended partial response class IV format. This data stream may then be passed to a detector circuit which interprets customer data, and attempts to recognize a sync word.
The operation of both the ADC and the detector are timed with respect to a receiver clock. Therefore, as part of preparing to receive customer data, the receiver clock must be phase-locked with a clock utilized to transmit or record the data stream. The process of phase-locking requires a finite time period
The problem in the prior art of detecting byte sync lies in the fact that a circuit for indicating byte sync is monitoring the output of a detector for the appearance of a sync word, which, when received, indicates that byte sync exists. If the waveform is poorly equalized, or, where a time-varying detector is utilized, the time-varying detector has not been correctly set, the output from the detector will be unreliable. Thus, a byte sync derived from the output of a detector operating in an unreliable condition cannot be used to trigger the training of an adaptive equalizer with respect to a training waveform, or to set the timing for a time-varying detector. If a time-varying detector is utilized, the circuit for indicating byte sync may not rely on the output of the detector at all, because the output of such a time-varying detector is not valid until the detector has received an indication of byte sync.
Additionally, the process of detecting byte sync takes place during the reception of a number of data bits, as the training portion concludes and the sync word is being received. During this process, it is important to distinguish received bits that belong to the training portion of the incoming waveform from those bits that belong to the sync word portion. As the process receives information from the incoming waveform, it is important to avoid a determination that byte sync exists before all bits, or substantially all bits, of the sync word have been received. The indication of byte sync when byte sync does not exist may be called indicating a false byte sync condition. A false byte sync may be indicated if the sync word portion does not significantly differ from, or does not contrast with, the training portion of the incoming waveform. Such a difference or contrast between the two numbers may be referred to as the distance between the two numbers.
Therefore, it is desirable to provide a method and apparatus for identifying the first bit of the first word of data in a data stream, wherein the method and apparatus is not easily affected by input signal degradation due to media noise, thermal noise, amplitude variation, electromagnetic interference, phase misalignments, misequalization, transients, and D.C. offsets. Furthermore, it is desirable to provide a method and apparatus for identifying the first bit of the first word of data in a data stream that does not rely upon the output of a data detector.